1. Field of the Invention
The present invention relates to an inrush current protection circuit and an application thereof, and more particularly to an inrush current protection circuit which applies to a load an inrush current initially generated when an alternating current is applied thereto and having a magnitude thereof reduced, thereby protecting the load from electrical damage, and to an application of the circuit to a power source of a microwave oven.
2. Description of the Prior Art
A high voltage is generally needed to drive a magnetron of a conventional microwave oven, and a high voltage transformer is used for the high voltage.
A drive voltage which is supplied to the high voltage transformer of the microwave oven and to a load connected to a second coil thereof should be stable. In relation to a drive power supply of such a microwave oven, technologies for supplying stable powers to loads are disclosed in U.S. Pat. No. 4,798,927 issued to Accuse Kaminaka, U.S. Pat. No. 5,250,774 issued to Kong-Keen Lee, and U.S. Pat. No. 5,625,520 issued to Bung Ap Lam.
In the U.S. Pat. No. 4,798,927, Kaminaka suggests a technology for selectively and intermittently supplying an electric current to microwave and heat ovens by appropriately combining switching means corresponding to cooking modes, thereby maintaining a temperature of the ovens at a predetermined value and also making it possible to cook food without any extension of cooking time. In the U.S. Pat. No. 5,220,774, Lee suggests a technology for preventing instability of output voltage due to LC resonance between a high voltage condenser and a secondary winding of a high voltage transformer, and thereby preventing damage of a magnetron due to a high voltage and providing a stable power to the magnetron, by interposing a diode between the secondary winding of the transformer and the condenser for driving the magnetron and thus variably inducing the output voltage of the transformer depending on a feedback voltage. In the U.S. Pat. No. 5,625,520, Lim suggested a device for preventing a high voltage transformer from being overheated. Lim's device directly senses a temperature in a second coil of the transformer and uses a thermostat to control a power supply to a first coil of the transformers according to the detected temperature.
On the other hand, there is a need to intermittently supply a drive power to a high voltage transformer in order to prevent a magnetron from being overheated. Particularly, in a power source of a mechanical microwave oven, a drive power is intermittently supplied to a first coil of a high voltage transformer according to a selected cooking time and a high frequency output level. However, the intermittent supplying of the drive power is accompanied by an inrush current, which applies electrical impacts to the power source and generates flickering in the power source. Therefore, the high voltage transformer is required to be provided with a stable voltage, while minimizing variations in the drive power source.
Referring to FIG. 4, a drive mechanism of a power source in a conventional microwave oven is shown. After a food to be cooked is placed in a cooking chamber and then first and second doors are closed, switches 110a ad 110b of the doors are turned ON. In this state, a user uses an output regulating knob (not shown) to select an output of a magnetron 126 and uses a timer knob (not shown) to set a cooking time. Then, a timer switch 104 is turned ON to drive a timer motor 106, and in turn, to drive a cavity lamp 142, a turn table motor 144, and a cooling fan motor 146. A variable power control switch 108 is periodically turned ON and OFF according to control signals of the timer motor 106 to intermittently operate the magnetron 106. The intermittent power supply to the magnetron prevents the food to be cooked from being overheated.
However, an output wave of the control switch 108, immediately after the control switch 108 is switched ON, includes an inrush current of over a predetermined magnitude as indicated by "A" in FIG. 3. In case such an inrush current is directly applied to a first winding 128 of a high voltage transformer 120, an electrical impact which is generated by the inrush current can impair endurance of the device and can also cause flickering, which undesirably affects a health of a human body and operations of peripheral devices. For this reason, in European nations, a regulator named "Flicker" is provided to regulate a voltage variation of an input power source at an amount proportional to the amount of the inrush current, and to strictly control product standards of electrical devices. The inrush current should be effectively controlled to satisfy the product standards.
In order to prevent the inrush current, as shown in FIG. 4, there are provided a fuse 114, a cement resistor 112 connected in parallel to the fuse 114, an AC relay 116, and a bypassing switch 110 associated with the AC relay 116. When an energized current is applied to the AC relay 116 and the AC relay 116 sends an operation signal to the switch 110 after a delay which is a self-response time needed to energize an energizing coil. As a result, the inrush current generated immediately after the control switch 108 is turned ON is flowed to the cement resistor 112 and damped during the response time, and then is bypassed to the switch 110.
The fuse 114 is provided to prevent the AC relay 116 from being unintendedly turned ON due to an abnormality thereof or to a delayed response time thereof. An abnormality of the AC relay 116 can cause the cement resistor 112 to be overheated, thereby badly affecting wires and moldings around the wires and occasionally starting a fire.
The response time of the AC relay is, in general, approximately 6 ms, and is 10 ms in a slow acting type AC relay in which the contact point structure thereof is modified. That is, the bypassing switch 110 of the AC relay 116 remains OFF for 10 ms from the time that the inrush current starts to be applied to the AC relay 116, and then is turned ON. A measured output current of the control switch 108 illustrates that, as indicated by "A" in FIG. 3, an inrush current of over a predetermined magnitude is generated during at least one period of the AC power source. For example, the period for a 50 Hz power source is 20 ms. Therefore, the time needed for the inrush current to pass the cement resistor 112 should be at least 20 ms in order to prevent bad influences of the inrush current and thereby supply a stable power to the high voltage transformer. However, the maximum self-response time of a slow acting AC relay is restricted to 10 ms due to its point contact structure, and thus the inrush current cannot be effectively prevented.
Moreover, since the AC relay 116 has four tap terminals, terminal inserting processes are required to assemble it. Further, the cement resister 112 is a terminal type of two tap terminals and is mounted on the bottom surface of an electrical device chamber by screws, so terminal inserting processes and screw threading processes are required. The AC relay 116 is especially used in a mechanical microwave oven in which a DC power source cannot be used, and is generally expensive. Further, the fuse is effective in preventing a fire which can be started by an abnormality of the AC relay, but has a disadvantage in that it needs to be replaced after use.